This invention relates to solid state area and line imaging devices.
There is currently a widespread attempt to develop area and line imaging devices which are all solid state to replace present devices employing an electron beam as the scanning element. Recently, two new forms of semiconductor information storage devices have been proposed which would permit economical manufacture of such imaging devices.
One of these forms of information storage devices has come to be known as the Charge Coupled Device (CCD). (See, for example, Tompsett et al, "Charge Coupled 8-Bit Shift Register," Applied Physics Letters, Vol. 17, No. 3, pp. 111-115.) The device comprises an array of metal electrodes disposed upon an insulating layer which overlies one surface of a semiconductor medium. The device stores mobile charge carriers in the medium in potential wells created under the biased electrodes and transfers the charge by creating a succession of potential wells along the semiconductor surface when series of the electrodes are successively biased. The charge carriers may be generated in many ways. One method is to generate hole-electron pairs in the medium by photon absorption. Minority carriers will then be swept into the potential wells formed under the biased electrodes in proportion to incident light and can be read out merely by sequentially biasing the proper series of electrodes. (See U.S. patent application of W. S. Boyle and G. E. Smith, Ser. No. 11,541, filed Feb. 16, 1970 now abandoned in favor of continuation-in-part application Ser. No. 196,933 filed Nov. 9, 1971 now U.S. Pat. No. 3,858,232 and assigned to the present assignee.)
The other form of information storage device which may be used for imaging applications has come to be known as the Bucket Brigade Device (BBD). (See, for example, U.S. Pat. No. 3,660,697, filed Feb. 16, 1970, issued May 2, 1972 to C. N. Berglund and H. J. Boll.) This device also employs an array of electrodes disposed on an insulating layer overlying a semiconductor medium. Here, however, diffused regions are provided in the semiconductor beneath each electrode and extend slightly into the area below an adjacent electrode in the charge transfer path. When an electrode is pulsed, the diffusion immediately under it is reverse biased and the channel between this diffusion and its neighbor is inverted. Thus, charge is collected and stored in the diffused regions in proportion to incident light as majority carriers and transferred through the channel regions between diffusions as minority carriers when the electrodes are successively biased. It can easily be seen therefore that the reverse biased diffused regions perform in the same way as the potential wells of the CCD during charge collection.
One of the difficulties associated with these devices as proposed is a lack of sharp resolution. That is, since light is continually incident on the medium, additional carriers are collected during readout causing a smearing of the image.
Since both the CCD and BBD devices store and transfer charge carriers in a semiconductor medium by means of appropriately biased electrodes, the term "Charge Transfer Device" (CTD) has become the accepted generic description of the two types of devices. The term "localized integration site" is the generic description for the regions of charge collection, whether the potential wells of the CCD or the equivalent diffused regions of the BBD. These terms will be so used in the context of this application.